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Passivating plain carbon steel

An ongoing discussion from 2001 through 2015 . . .

(2001)

Q. I am having trouble convincing a customer (computer hardware manufacturer) that you only passivate stainless steel and not normal carbon or alloy steels. How can I proceed to verify this with them. Is there an exact specification I can go to. This is all about having some tools clean enough for a computer clean room environment.

Herb Earl- Phoenix, Arizona

(2001)

A. Your customer may be confusing "passivating" with "passive". Many times a steel part needs to be activated for plating or finishing because it has become passive. Passivating for stainless means to remove the iron from the surface and create a nickel and chrome- rich surface layer that is not as subject to rust (only iron "rusts"). Let your customer know that a steel part will always have a propensity to rust - even if it has become passive, because the iron is always exposed to the atmosphere. Hope this helps.

Dan Brewer chemical process supplier - Gurnee, Illinois

(2001)

A. Herb:

You cannot "passivate" carbon steel. Passivation is removing the free iron and other exogenous materials from the surface so that a good passive layer can be formed. In the case of carbon steel, this is not possible. After cleaning with a good cleaner, there are a variety of coatings that can be used for carbon steel to provide a corrosion resistant layer, but passivation is not the correct process.

A. Passivating carbon steel. If carbon steel could not be passivated then there would be no boilers made of carbon steel. Boiler steel passivation occurs by the Schikorr reaction at about 300 °C or 30 bar pressure. A protective "passivating" magnetite layer, a few microns thick forms by a reaction between clean, pickled steel and water. The reaction lowers the pH of the boiler water. In practice, newly pickled boilers are subjected to controlled passivation using very small quantities of chemicals to control pH and prevent re-dissolution of new magnetite. The thickness of this magnetite layer will continue to grow at a decreasing rate until it hinders heat transfer. The boiler is then again chemically cleaned.

The chemical cleaning itself, by pickling, strips oxide layers from the boiler surface, leaving the steel active and prone to immediate oxidation (rusting).

To stop this "flash-rust" disturbing the Schikorr reaction, the pickled boiler receives an initial "temporary passivation". This involves dissolution of the flash rust in citric acid, followed by raising the pH to the range 8.8-9.5 using ammonia, which keeps sequestered iron in solution. Once the pH is correct, an oxidizing agent, for example sodium nitrite is introduced. This reacts with the iron in the steel to produce a very thin (40-100 Angstrom) layer of Fe2O3, which, when formed under these circumstances, inhibits flow of ions through it, thereby passivating the steel. This passivity is easily destroyed and is therefore temporary in nature (unlike passive oxides formed on stainless steel).

For carbon steel items to be used for everyday use, coatings are indeed usually applied. Some coatings involve chemical reactions in which iron from the steel plays a part (such as zinc phosphating), while other, usually more durable coatings physically attach themselves to the steel, for example paint. It is common that physically adherent coatings are applied to steel that has been temporarily passivated by, for example the formation of iron phosphate. Such phosphates are formed by dipping pickled steel into hot dilute phosphoric acid solutions followed by exposing them to air to complete the process. An example of such a process is phosphating and painting of car body parts.

For tools to be used in clean-room environments preference should be given to quality stainless steel implements.

Trevor Tipton- Singapore

(2002)

A. I think Trevor has confused conversion coatings with passivation. To begin with, carbon steel boilers don't rust because the pH of the water is kept above the oxidizing point of steel which is about 8.2 - 8.3. The magnetite coating is a chemically induced conversion coating
(also called black oxide), that if left in a pH environment below 8.2 will start to rust. If proper boiler chemistries and and periodic blowdown schedules are followed, there should be no buildup requiring chemical stripping of the boiler tubes. I have seen boilers that have been online for 10 years and more without needing a chemical strip.

Parts that receive a phosphate treatment likewise are not immune to rust and cannot be considered passivated. This is another example of a conversion coating.

To close, passivation is accepted as being a theoretically permanent condition. Passive means to not be active enough for the next finishing step.

Dan Brewer- Gurnee, Illinois

(2007)

Q. Dear Trevor Tipton and Dan Brewer:

I liked your responses and would like to ask what is a good reference to read about the passivation of Boiler interiors?

and is it possible to obtain magnetite based solely on NaOH/Na3PO4 and ammonia dosing for surfaces below and above 180 °C and pressures above 100 bar?

best regards

rami cremesti- galabovo, Bulgaria

March 27, 2009

A. Passivation is subjected to lot of conditions. Say when you talk about a boiler, carbon steel tubes in generating section will have a reduced oxide in a state of magnetite. it remains intact only when you treat the water and chemically condition the same. demineralised water, free from oxygen and chloride,pH elevated to 9.0 to 9.5, free from free caustic etc. it is non impervious to ionic intrusion.
Whereas if it is gamma ferric oxide, it will remain intact and it's P.B ratio is above 1.0 it is impervious due to structural integrity and natural oxide growth rate during regular operation is retarded and beneficial in all respects.

thiruvenkatachari sampath- Chennai,Tamil Nadu, India

August 27, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have heard a lot about stainless steel passivation using various chemicals. But is it possible to passivate carbon steel piping using sodium hydroxide, hydrochloric acid, soda ash, Rodine-213 (corrosion inhibitor) and glassy sodium meta phosphate.
Can you suggest alternative for the same.

Regards,

Hemant Banait consultant - New Delhi, India.

May 9, 2010

A. Passivation is a state wherein we protect it from re-oxidation.
A chemically active surface is more prone to attack due to its active state after immediate chemical cleaning. It is important to make the metal passive to prevent the re-oxidation and there by preventing further corrosion
normally the medium is switched to an alkaline state after an acidic reaction with an oxidising agent to get the passivation layer on to the surface.

A. Hi, Bart. Thanks, but yes and no. The primary issue here is the word 'passivate', which is just slang, not a specification, and slang can have a very broad meaning depending on who is slinging it, such that nearly anything can be 'passivated': zinc and electroless nickel surfaces are 'passivated' with chromate conversion chemistry; steel surfaces are said to be 'passivated' with phosphatization; and the phosphatization in turn is said to be 'passivated' with a final chrome or chrome-free rinse.

But there is also a narrower meaning of passivation, referring to processes that react with the chromium content of stainless steel and can therefore only apply to chromium-bearing stainless steels, not carbon steels.

This narrower interpretation does have some validity: note that when you consult a chart of the galvanic series, you find that active stainless steel is very different from passivated stainless steel, whereas you will see no similar distinctions for "active" and "passive" plain steel. Note also that there are recognized "standards" for "Passivation of Stainless Steel" (e.g., ASTM A380, ASTM A967, ASTM B912, AMS2700), but no such standards for passivation of carbon steel.

As a warning to readers, do not attempt a nitric acid "passivation" of plain steel; it can evolve toxic quantities of NOx.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

January 1, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Is it possible to passivate BI pipes? If it's possible, what is the recommended acid for passivation? Is phosphoric acid applicable? Thank you.

Liz Bargola - Philippines

January 6, 2013

A. Hi Liz. It is certainly possible to treat rusty steel with phosphoric acid to convert the fluffy red rust to the more stable black rust. Whether this should be called 'passivation' is a semantics argument over the use of that word rather than a technical issue. Some people consider the word 'passivation' to have a very specific meaning that applies only to the processes used to remove plain steel contamination from stainless steel.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

July 10, 2013

Q. I've seen lots of conjecture about passivating metals.
If anyone has good information I would like to know:
General Question: What is a good global definition of passivating and passive?
Specific question: Is it possible to passivate carbon steel in aqua ammonia to prevent further corrosion at ambient temperatures.
Thanks.

M. Hubbard - Denver, Colorado, USA

July 12, 2013

A. It is possible but that is an old fashioned way, and not as effective as other methods such as Parkerizing would be better. If it's a prototype you can try it. If the application does not contain an aggressive environment or a lot of H2O then you might get away with bluing or using the method you mentioned, it won't hurt, it will help a little to use an alkali solution.

Blake KneedlerFeather Hollow Eng. - Santa Clara, California

July 12, 2013

A. My thought is that it would be a neutralizing rather than passivation step.
Carbon steel does not lend itself to traditional acid passivation.
A dip in dry to the touch oil would probably have a longer effect than the ammonia treatment. Possibly use both.

James Watts- Navarre, Florida

A. Hi M. As this page notes several times, "passivation" does have a good definition when it is applied to stainless steel, and ASTM & AMS and other specs provide those careful definitions you seek. But the word is also used in slang fashion for other situations and there is no way anyone can delimit slang; this is not "conjecture" :-)

We can also claim that a "tank" is an armored fighting vehicle and then write careful definitions for how thick the armor needs to be and what material specifications it must meet before it can be called "a tank", but that won't stop teenagers from calling an old station wagon "a tank", let alone stop people from calling their aquariums, hot water heaters, propane cylinders, and gasoline reservoirs "tanks" :-)

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

July 30, 2013

A. M.,
Passive is actually a chemist's term, it is the opposite of active. Active refers to atoms or molecules that "want" to react with something. Taking metals as a pertinent example, metal as we think of it has an electron valence of zero, which is to say the number of electrons in orbit is equal to the number of protons in the nucleus. However most atoms, including metals, "prefer" to have a different number of electrons, in the case of metals they want to give some away and become a positive ion in solution or become bonded to the atom that it gave its electrons to. Surface oxidation is the most common route to this, for instance if you cut a block of aluminum in half you can see the slight color change across the fresh surface as oxygen gas swoops in and quickly forms a layer of aluminum oxide. The new oxide layer is "passive" because it is happy and no longer want to react to anything, and as a bonus it seals off the zero valence aluminum underneath from being able to react with anything.

This is all fine and good for the majority of nonferrous metals, but when iron forms iron oxide it is not a nice and pretty layer, it turns to rust and falls off, exposing new iron to new oxidation. Common methods to combat this are coatings of another material, such as paint, powder coat, zinc (galvanizing), a chromate conversion coating, etc. There are also stainless steels, which use chromium to form a chromium oxide layer over the iron, protecting it the same way the aluminum oxide prevents further reaction in the underlying aluminum metal.

Anything that renders your surface less likely to chemically react with materials in the environment can legitimately be called "passivating".

What is "sequestered iron"?

July 20, 2014

Q. Hi.
What is "sequestered iron"?
Iron II and III in complex with citric acid is "sequestered iron"?
After the pickling phase, the steel surface will be very reactive and oxygen will cause flash rust to form.
What is flash rust? Fe2O3?

If flash rust is Fe2O3, why we must remove flash rust before "temporary passivation"? we create Fe2O3 in temporary passivation.

hasan eghbalipower plant - mashhad, khorasan, Iran

July 2014

Hi Hasan. I am not a chemist, and hope to be corrected if I'm seeing it wrong, but your question is partially technical and partially semantics. According to Wikipedia, sequestration and chelation are much the same thing, but others might use "sequestration" in a looser sense where the iron is tied up or complexed in ways that may not meet the actual definition of chelation -- for example, iron can be complexed with cyanide ("prussian blue") and it is probably proper to say that the iron is sequestered.

I think any condition which keeps more iron in solution than the pH and alkalinity alone would dictate probably represents sequestration.

Flash rust is red, so I'd say it is Fe2O3. I believe the idea in pretreatment is to remove the rust and then get on with the treatment process immediately rather than letting flash rust form.

A. Indeed, "sequestered" isn't really the preferred term by chemists but I'm pretty sure as a colloquial it means the same thing as "chelated". Wikipedia seems to agree.

There are a few different varieties of iron oxide, depending on the oxidation state of the iron atoms, or the ratio of iron atoms to oxygen atoms present. I always forget which is which, but black iron oxide tends to be pretty stable while red iron oxide is the rust that eventually leads to progressive deterioration of the metal.

I have heard of "temporary passivation" of carbon steel in regards to boilers -- I'm not overly familiar with the process but in general any coating that prevents oxygen molecules from contacting zero valence iron is getting you what you need, for as long at it's intact.

Q. Dear All,
I just have gone through with interest the arguments put forward by the august participants in the Q & A column. It is not clear why it is said that carbon steel does not need passivation. All the water tubes of sub-critical boilers are of carbon steel and all the top notch manufacturers of such boilers recommend chemical cleaning of the boiler tubes by alkali boil out to remove oil, grease, loose debris etc, then rinsing, thereafter acid pickling to remove rust, mill scale etc. This is followed by DM Water rinsing, then citric acid pickling and thereafter, neutralization by raising pH with sodium nitrate solution in a sequential manner. After this, passivation of the nascent iron surface is effected by ammonia and hydrazine solution having pH 10. This helps in the formation of a passive Magnetite (Fe3O4) layer which prevents rerusting of the tube internal surfaces. This magnetite layer can be maintained if the pH of the Boiler water is maintained between 8.5-9.5.

My understanding is that the alkali boil out procedure alone cannot remove the rust, mill scale, etc. from the tubes and form the passivated layer.

I would be thankful if someone can inform that alkali boil out alone can remove rust, mill scale and form the passivation layer. Also I would welcome an explanation how the Tri-sodium phosphate and Sodium Hydroxide are instrumental in forming the passivation layer.

Thanking you all,

Dhruba Jyoti Gupta - CALCUTTA,WEST BENGAL,INDIA

November 2014

Hi Dhruba. I don't think anyone claimed that carbon steel "does not need" corrosion-proofing. I think what they claimed is that carbon steel does not use or benefit from the processes that are used to passivate stainless steel, and that some people reserve their use of the word 'passivation' to refer to those carefully defined and specified processes for stainless steel.

Sorry, I am not familiar with the protocol for preparing boiler tubes for service beyond what you read on this page.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

June 24, 2015

Q. Is it possible to use Gaseous Oxygen as the oxidizing agent post mill scale removal with citric acid? The short answer of just "NO" or "YES" is not what I am looking for, but rather why you cannot or can. Traditionally it has been a "NO" and to just use Sodium Nitrite to solution, never an explanation of why you cannot use gaseous oxygen.

Aaron Rook - Jacksonville, Florida USA

June 26, 2015

Aaron,
I may be missing some context to your question. What metal and for what purpose do you need oxidation? Gaseous oxygen, or just plain air since there's plenty of gaseous oxygen in there, is splendid for converting active zero valence metal atoms into metal oxides. It does vary a little depending on the metal, for instance if you are talking about iron it helps to have some moisture involved. If you are talking about converting zero valence metal atoms into metal ions via oxidation, you will need an aqueous solution containing an oxidizing agent, which can be but is not limited to acids.

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